1. Field of the Invention
The present invention relates to a multigas sensor and multigas sensor device used for measuring the concentrations of nitrogen oxide and ammonia contained in a gas to be measured.
2. Description of the Related Art
As a technique for purifying nitrogen oxide (NOx) contained in an exhaust gas discharged from an internal combustion engine such as a diesel engine, recently, a urea SCR (Selective Catalytic Reduction) system has attracted attention. In the urea SCR system, ammonia (NH3) and nitrogen oxide (NOx) chemically react with each other so as to reduce the nitrogen oxide to nitrogen (N2), thereby purifying nitrogen oxide contained in an exhaust gas.
In the urea SCR system, there is a possibility that, when the amount of ammonia supplied to nitrogen oxide is excessive, unreacted ammonia which remains contained in the exhaust gas is discharged to the outside. In order to suppress such discharge of unreacted ammonia, a multigas sensor which includes a sensor element for measuring the concentration of ammonia contained in an exhaust gas, and which can measure the concentrations of a plurality of kinds of gasses is used in a urea SCR system (for example, see JP 2011-075546A (Patent Document 1) and US 2010/0161242A (Patent Document 2)). In the urea SCR system, the amount of ammonia to be used in the reduction of nitrogen oxide is adjusted so that the concentration of ammonia to be measured in the multigas sensor, i.e., that of ammonia contained in the exhaust gas, is within a predetermined range.
3. Problems to be Solved by the Invention
The multigas sensor disclosed in Patent Document 1 is configured by disposing an NH3 sensing cell in a NOx sensor. In the multigas sensor, it is possible to obtain only measurement signals which are output from the NOx sensor and the NH3 sensing cell, respectively. In the case where three kinds of gases such as nitrogen monoxide (NO), nitrogen dioxide (NO2), and ammonia (NH3) are to be measured, therefore, it is difficult to correctly calculate the concentrations of the three kinds of gases, and there is a possibility that only concentrations with insufficient accuracies can be obtained.
The multigas sensor disclosed in Patent Document 2 is configured by combining together an NH3 sensing cell, a NO2 sensing cell, and a NOx cell. The multigas sensor calculates the concentrations of ammonia, nitrogen monoxide, and nitrogen dioxide, respectively based on measurement signals of the NH3 sensing cell, the NO2 sensing cell, and the NOx cell.
In the multigas sensor disclosed in Patent Document 2, however, the electrodes of the NH3 sensing cell and the NO2 sensing cell are formed by different materials. Therefore, it is necessary to take a countermeasure for ensuring the reliability of both the NH3 sensing cell and the NO2 sensing cell. Namely, the electrodes may deteriorate in different degrees. When the use period of the multigas sensor is long, therefore, a problem arises in that the deterioration degree of the electrodes largely differ from each other, and the accuracy of the measurement of the concentration of a certain gas species is impaired.